This invention relates to load pull testing of millimeter power transistors using automatic microwave tuners in order to synthesize reflection factors (or impedances) at the input and output of said transistors.
A popular method for testing and characterizing microwave components (transistors) for high power operation is “load pull” and “source pull”. Load pull or source pull are measurement techniques employing microwave tuners and other microwave test equipment. The microwave tuners in particular are used in order to manipulate the microwave impedance conditions under which the Device Under Test (DUT, or transistor) is tested (FIG. 1). A load pull setup comprises, among other parts, a signal source (1), a directional coupler (2), one or two impedance tuners (3, 4), two power meters (5, 6) and a DC power supply (7). It also comprises a test fixture to mount the DUT; at millimeter wave frequencies (typically above 26 GHz) waveguide impedance tuners [4] and components are used due to their lower loss, easier manufacturing and more steady performance. At these frequencies for testing chips on wafer (12) said test fixture is replaced by a pair of waveguide-wafer probes (8, 9 and FIG. 4). Said probes also include DC biasing networks (13), because the waveguide sections themselves do not allow the flow of DC current. The tuners themselves (3, 4) must be mounted on micro-positioners (10, 11) which allow exact positioning of the wafer probes (8, 9), which are rigidly connected to the tuner bodies, in order to make safe and repeatable contact with the chips on the semiconductor wafer (12).
A popular family of electro-mechanical tuners, the “slide-screw tuners”, use adjustable probes (slugs) inserted into the transmission media of the tuners. Said transmission media in millimeter-wave frequencies (typically, but not exclusively, frequencies >26 GHz) is a slotted straight section of waveguide (14) in which a metallic probe (pin) (15) is inserted and creates a capacitive load. (FIGS. 2a, b). When the probe (15) is inserted further (18) into the slot (16) of the waveguide the capacitive coupling increases and so does the reflection factor. When the pin is moved along the slot and parallel to the axis of the waveguide (17) the phase of the reflection factor changes. This way a major area (19) of the Smith chart can be covered (FIG. 3). The shadowed area (19) is called the “tuning range” of the tuner.
For testing semiconductor chips on wafer, “wafer probes” must be used, which are able to transfer RF energy to the chips in a reliable and repeatable fashion (FIG. 4). Since the chips are made using planar technology (all RF contacts are on the same side of the semiconductor wafer) the predominant transmission line configuration is Coplanar Waveguide (CPW) [1]; the probe tips are therefore designed using the same technology (CPW). A schematic view of said probe tips is shown in FIGS. 8 and 10. The dimensions are very small. The typical distance between the probe tips (20) is of the order to 100 microns. The probe tips have some elasticity. When pressed against the semiconductor surface (21) they will bend slightly and compensate for a certain angle Θ (Theta) between the plane (22) defined by the three probe tips and the plane (23) of the semiconductor surface (29). However, in many cases this elasticity is not sufficient to compensate for a probe planarity misalignment Θ. This effect is called the Theta angle misalignment (25). There exist techniques allowing the probes to be rotated to correct for the misalignment, but those are applicable only if the probes are connected to a flexible coaxial cable and used to test various RF data, (FIGS. 5, 6). Correcting for the Theta misalignment is particularly difficult when the probes are mounted rigidly on bulky impedance tuners (24), which cannot be tilted (FIG. 8). Using flexible cables to connect impedance tuners with wafer probes allows correcting the Theta angle by rotating only the wafer probes, but the insertion loss of the cables reduces the tuning range (19) of the tuners at the DUT reference plane and needs to be avoided (FIG. 3).
When using waveguide millimeter-wave tuners on-wafer, waveguide wafer-probes must be connected to the tuners (FIG. 8). The wafer probes themselves are use coaxial pieces of cable of very small diameter (34a) which are connected to rigid 90 degree waveguide bends (FIG. 4). FIG. 7 shows the assembly of such a wafer probe with a tuner: The flange at the end of the rigid waveguide bend (26, 34) is attached to the waveguide flange (27) at the test port of the tuner (35). This creates a very low loss rigid connection between tuner and wafer probe. The three end tips (20) of the wafer probes at the end of the coaxial cable (34a), in a typical GSG (Ground-Signal-Ground) coplanar electromagnetic wave configuration (FIG. 8) are made to lie on the same horizontal plane (22), in order to establish safe and repeatable contact with the semiconductor chips. If the wafer probes are rigidly connected to the tuner box (24), then rotational adjustment (28) of the probe tips is only possible by rotating (29) the whole tuner (24). This invention describes an alternative and simpler solution.
In case of a waveguide tuner, which has a test port in form of a waveguide flange (FIG. 7), said rotational adjustability can be provided by elaborated mechanical arrangements (30) of the whole tuner-probe assembly rotating (29) around an axis (FIG. 8). In this case because the center of rotation of the tuner (24) is different than the point of the wafer probe tips (20), then said probe tips will rotate on an arc (28); this means that the adjustment procedure is tedious and complex, since, every time the angle Θ is adjusted the probe tip position changes and it may move out of the scope (31) of the observation (32) microscope (FIG. 7), in which case a new XY positioning of the whole tuner-probe assembly is required, to bring the probe-tips back into focus, test the contact, re-adjust the Θ angle and so on. This can be a frustrating and lengthy experience. In short, a setup involving wafer probes must provide for some rotational adjustment of the wafer probe tips in order to fit perfectly on the chip pads (FIG. 8). Rotational adjustability Θ accepted as adequate at this point of time is ±3.5°.